Wind turbines are used to convert wind energy to electrical energy in a clean and efficient way. In a wind turbine a rotor comprising rotor blades drives an electric generator, either directly or by means of a gearbox. The alternating current (AC) frequency that is developed at stator terminals of the electric generator is directly proportional to the speed of rotation of the rotor. The voltage at the stator terminals also varies as a function of the rotational speed of the generator. For an optimum energy capture, this rotational speed varies according to the speed of the wind driving the rotor blades. To limit the energy capture at high wind speeds and to avoid a damage of the rotor, the rotational speed of the electric generator is controlled by altering the pitch angle of the rotor blades.
An adaptation of the variable voltage and frequency of the electric generator to a nominally fixed voltage and frequency of a power grid is typically achieved by a power converter. A power converter typically includes a generator bridge, which in normal operation operates as an active rectifier in order to supply power to a direct current (DC) link. The generator bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy. A power converter typically comprises two network bridges, wherein a first network bridge converts the AC power signal provided by the generator to a DC power signal and a second network bridge converts this DC power signal to an AC power signal, which in voltage, frequency and phase angle is matched to the power grid. Since a power converter always accomplishes a frequency conversion, such power converters are often also denominated as frequency converters.
However, in practice there always exist at least some disturbances on an electric signal being present at an electric power connection line between the wind turbine(s) and the power grid. These disturbances are in particular harmonic disturbances, which may be caused for instance by the switching frequency of a PWM modulated power converter or of passive rectifier bridges, system resonances, arc furnaces . . . etc. However, disturbances are also caused by electric equipment being connected to the power grid. In this respect electric equipment may be any electric system or electric device, which is connected to the power grid. In particular, electric equipment includes electric consumers or user equipment and electric power generating systems such as any type of electric power plant (e.g. one or more wind turbines), which feeds electric energy to the power grid.
It is mentioned that disturbances can also be caused by non-idealities in the power system itself, e.g. saturation in the power system transformers.
The reliability and efficiency of the power grid as well as the connected equipment is strongly dependant on the harmonic distortion in the system voltage of the power grid. Therefore, power grid operators are required to ensure a reliable delivery of power with a quality that meets certain standards to ensure an untroubled operation of any type of electric equipment being connected to the power grid. Therefore grid operators are taking large measures to keep the harmonic distortion within predefined levels. This holds in particular for the harmonic distortion caused by power converters or frequency converters. However, it should be understood that of course all harmonics in the power grid are unwanted and might course problems in the system and/or in connected electric equipment. No distinction is made between how these harmonics are generated.
At present, the main focus related to the handling of the harmonic emission from power converters is directed on the most significant harmonics of a switching frequency, their sidebands and multiples of these sidebands. It is known to suppress these harmonic frequencies by means of special filters, different switching patterns, phase shifting between different switching pattern, etc. Techniques also exist, by which individual harmonics can be cancelled by the application of algorithms to select appropriate PWM switching patterns.
EP 1 995 863 A2 discloses a method for controlling a plurality of power converters that can be used to interface to a supply network or a power grid. Each power converter includes a network bridge operating in accordance with a PWM strategy, which has the same switching period and which causes at least one unwanted harmonic in the voltage of the power grid. The method includes the step of providing the switching period of the PWM strategy of each network bridge with a different time offset relative to a time datum such that the at least one unwanted harmonic in the supply network voltage is at least partially cancelled.
Generally, lower order (frequency) harmonics are not considered as to cause significant distortion problems. However, where problems in connection with lower order harmonics are expected, it is known to use so called park level filters in order encounter such problems. In this respect park level filters are high power filter devices, which suppress certain harmonics in a common electric power output signal being provided by two or more wind turbines or by a whole wind farm or wind park.
For characterizing the harmonic emission from an electric power generating system being connected to a power grid by means of a power converter (or frequency converter), there is typically made the assumption that the electric currents measured at the terminals of the power converter are all generated by the power converter itself. This assumption, which can be seen as a consequence of not taking into account the harmonic disturbances (background harmonics) which are already present in the power grid, has inter alia the drawback, that an active filtering cannot be incorporated in the power converter, because it is presently not possible to distinguish between harmonic disturbances caused by background harmonics within the power grid and harmonic disturbances caused by the power converter.
With an increasing amount of electric power generating systems, in particular wind turbines, being connected to the power grid via power converters, the demand for techniques to control- and/or to analyze the harmonic emission from such electric power devices is becoming increasingly important.
There may be a need for improving the analysis of electric disturbances at the connection between an electric power generating system and a power grid in particular with respect to the origin of the electric disturbances. Further, there may be a need for mitigating such electric disturbances in order to improve the electric quality of a power grid.